fix: use dorny/paths-filter to include/skip a job with a required status.

[ericswanson-dfinity]

Overview

I can't merge #645, because GitHub says I am missing a required status. What status that might be isn't obvious.

GitHub's docs say:

Warning: If a workflow is skipped due to path filtering, branch filtering or a commit message, then checks associated with that workflow will remain in a "Pending" state. A pull request that requires those checks to be successful will be blocked from merging.

For this reason you should not use path or branch filtering to skip workflow runs if the workflow is required. For more information, see "Skipping workflow runs" and "Required workflows."

If, however, a job within a workflow is skipped due to a conditional, it will report its status as "Success". For more information, see "Using conditions to control job execution."

It turns out that "paths-ignore" doesn't solve the problem. See https://github.com/orgs/community/discussions/13690

This PR takes two jobs that report a required status, and skips them with a conditional. If this works out (can't test it against the real statuses until it's merged to master), I'll copy this to the other workflows that generate required statuses.

Here is the workflow working on this PR: https://github.com/dfinity/examples/actions/runs/7674715486/job/20919794867?pr=651

Here is the workflow skipping both jobs in another PR: https://github.com/dfinity/examples/actions/runs/7674641514/job/20919580001?pr=650

Considered Solutions
I considered running the required jobs unconditionally, which would have been simpler, but also would have run tests unnecessarily.

Considerations
This will require a runner to evaluate the conditional, but it doesn't have to check out the repo.